Current spikes can be damaging when directing high voltage into machinery, transmission lines, or injector guns for accelerators. In the case of accelerators for example, high current spikes at high voltage can flash and damage the injector gun of the accelerator if the power supply is not properly current limited.
At the Thomas Jefferson National Accelerator Facility, the injector gun is designed to run at a 350 kV level. At startup, the injector gun must be conditioned for a period of time to ensure that the gun electrode is stable at high voltages and without current emissions. The voltage and current are monitored on startup and when the current fluctuations measure less than a few micro amps per 15 minutes, the gun electrode is considered stable at this voltage level. The voltage is then increased 1–2 kV and the process repeated until the voltage on the gun electrode is about 10–15% above the operating voltage. The time frame involved for the gun electrode to stabilize may be as long as two days. During the stabilization period, the current available to the gun electrode is limited by the large resistance of the conditioning resistor. After the gun electrode has been properly conditioned to a voltage 15% higher than the running voltage of 350 kV, the conditioning resistor is replaced by the running resistor, which has a much lower resistance.
At present, two separate resistors are used to control the current available from the high voltage power supply to the injector gun. A high value resistor is used to reduce the current available from the power supply to a low value to condition the injector gun. A low value resistor is then substituted for the high value resistor. The low value resistor is then placed on line allowing the high voltage power supply to provide higher currents when required by the injector gun for operations.
Unfortunately, the use of two separate resistors and the task of switching them causes a great deal of down time. High voltage power supplies for FELs may reach as high as 500 kV or higher. The separate resistors are bulky and must be secured in place between the power supply and the injector gun, within a surrounding jacket, which requires several hours of unproductive time.
What is needed therefore, is a dual resistor for high voltage applications that is capable of being switched from one resistance value to another without significant downtime or disassembly. The dual resistor must be capable of limiting the current available to the gun during high voltage conditioning and of delivering large current when required by the gun during operations. The dual resistor would be useful in starting up high power accelerators, high voltage transmission lines, or other high voltage equipment in which the current must be limited for conditioning or starting purposes.